The present invention is with respect to an instrument for measuring the hematocrit value between two electrodes, whose acting faces are in horizontal planes and which may be used for measuring the conductivity of blood.
An apparatus on these general lines is to be seen in the East German Pat. No. 78,307, in which the electrodes are so placed that the field lines are normal to the direction of sedimentation of the blood, this however being generally a shortcoming insofar as the impedance value is dependent on the degree of sedimentation and the instrument error becomes greater with every increase in the sedimentation rate. This is more specially a shortcoming in the case of pathological blood samples in which the aggregation rate is greater than in normal blood.
On working with such an instrument it is not possible to make a simple correction of a reading that has been falsified by the effect of sedimentation, because such a correction operation would be over-complex.
Furthermore under working conditions it is likely for the electrodes in the sample chamber not to be completely clean and to have layers of protein, more specially fibrinogen, formed on them causing false readings for the impedance and for this reason the hematocrit value itself.
A further shortcoming of this known instrument is the trouble in handling it under working conditions. When the laboratory technician puts the precision capillary tubes between the clamps there is a loss in part of the blood from the tubes so that the readings are inaccurate, such loss of blood in fact causing the presence of air pockets that are responsible for a great increase in the impedance value. And furthermore the lost blood has the effect of dirtying the apparatus and producing errors.
Moreover because the instrument is not accurately filled there will be a scatter of the readings taken dependent on such different volumes of filling, this being because the field line density in the blood is highly dependent on the volume filled.
In a further known instrument for measuring the hematocrit value, see German Auslegeschrift specification No. 1,229,760, the field lines produced by the electrode system are again normal to the direction of sedimentation so that there are likely to be the same trouble conditions in connection with taking readings as noted in connection with the apparatus of the East German patent.
Cleaning the electrodes is no simpler so that there are the same trouble conditions as with the first-noted form of instrument.
A further point in this connection is that the reading of the instrument is based on there being a linear dependence between the impedance and the hematocrit value that is not in fact in existence so that the translation of the readings is based on a false premise.
Lastly the sample temperature is not able to be kept constant because there is no thermostat, and this again is likely to make the readings inaccurate.
A further instrument for measuring the hematocrit level is to be seen in German Offenlegungsschrift specification No. 2,103,285, in which once again the field lines are normal to the direction the sedimentation so that the same troubles are noted hereinbefore are likely when taking readings. Furthermore large samples of blood are necessary for operation of the instrument. The electrodes may not be taken out for cleaning and for this reason there is likely to be trouble with contamination as noted earlier.
The U.S. Pat. No. 3,922,598 is with respect to a hematocrit measuring instrument in which again the field lines are normal to the direction of sedimentation, although in a further possible development of the instrument the electrodes are placed parallel over each other and may be dipped into the blood sample. In this case the depth of such dipping will not be constant because of the building up of sediment on the floor of the measuring chamber.
A further shortcoming is the overly large size of the volume in which measuring takes place on using the electrodes and there is trouble with cleaning the electrodes so that inaccurate readings are likely for the reasons noted hereinbefore.